Atom probes and three-dimensional atom probes are used for examining the structure of materials, particularly metals and semiconductors, at an atomic scale. Atom probes yield compositional information about the material, while three-dimensional atom probes also provide spatial information about the original atomic positions (e.g. resolving an image of a specimen).
A three-dimensional atom probe removes individual atoms from the surface of a needle shaped specimen with a small tip radius. The individual atoms are ionized and the ions are accelerated towards a position sensitive detector. The detector electronics measure the position at which each ion hits the plate and also calculates the mass/charge ratio of the resulting ion by measuring the time-of-flight (TOF) of the ion from the specimen to the detector.
Three-dimensional atom probes, and their relationship to atom probes generally, are disclosed in the publication ‘Atom Probe Field Ion Microscopy’ by M. K. Miller, A. Cerezo, M. G. Hetherington and G. D. W. Smith, OUP 1996, which is fully incorporated herein by reference.
EP1016123 A1 and EP0231247 A1, each of which is fully incorporated herein by reference describe atom probes and counter electrode configurations. EP1124129 A1, which is fully incorporated herein by reference, describes a device and method for two-dimensional detection of particles or electromagnetic radiation. U.S. Pat. No. 5,061,850, which is fully incorporated herein by reference, describes a high-repetition rate position sensitive atom probe. U.S. Pat. No. 5,347,132, which is fully incorporated herein by reference, describes a position sensitive detector. U.S. Pat. No. 5,440,124, which is fully incorporated herein by reference, describes a high mass resolution local electrode atom probe.
In order to identify the species of the atoms, the three-dimensional atom probe functions as a mass spectrometer. An electronic detector which measures both the position of the ion and its time of arrival in the same manner as a time of flight (TOF) mass spectrometer enabling elemental identification as well as their original position. In a voltage-pulsed atom probe the atoms at the apex of the specimen are field evaporated by applying short duration pulses to the specimen or electrode.
In a three-dimensional atom probe, ions from the specimen are emitted from an area of the tip which depends on the curvature. They are emitted approximately radially to the tip curvature. A detector is located typically 80 to 600 mm from the tip. The detector is typically square or circular, and has a width in the order of 40 to 100 mm.
In the conventional 3-dimensional atom probe a high voltage is applied to a needle shaped specimen in an ultra-high vacuum environment. A voltage pulse is then applied either to the specimen or to a nearby electrode which raises the electric field at the specimen tip beyond the threshold at which atoms of the specimen become ionized and are accelerated away from the specimen towards a position sensitive and time sensitive detector. The position and chemical identity of the original atoms are then determined and over many such events a three dimensional model of the specimen is reconstructed with atomic resolution.
In certain situations, the use of a voltage pulse can have a number of disadvantages. These include difficulties in analyzing poorly conducting or insulating materials and an inherent energy uncertainty which derives from the trajectory of the accelerating ion through a time-varying electric field coupled with an uncertainty of the precise moment of ionization.
As indicated above, the voltage pulsing technique is generally limited to highly conductive specimens. Energy compensation is often required to achieve higher mass resolution because of energy variations in the evaporated ions. This is particularly important where similar masses are present in widely differing abundances. There are at least three methods for energy compensation. One method involves post acceleration. In one application of this method a pulse is applied to an accelerating electrode which is positioned very close to the specimen. This means that absolute energy variations due to the pulse are reduced because a smaller pulse voltage can be used. The ions are then accelerated before passing into the mass spectrometer part of the atom probe. This means that the relative energy errors are made smaller. Full Width Half Maximum (FW0.5M) resolution is still limited to the order of 500 to 700 in practical designs. U.S. Pat. No. 5,440,124 describes a high mass resolution local electrode atom probe.
A second method is post deceleration. This can involve the use of a double electrode arrangement close to the specimen coupled with a fairly wide pulse and achieves a time focusing effect at the detector by decelerating early ions more than later ones. This is described in WO 99/14793, which is fully incorporated herein by reference.
A third method involves the use of a reflectron in an atom probe or three-dimensional atom probe, which provides very good energy compensation over the entire mass range. The reflectron effectively acts as an electrostatic ‘mirror’, and alters the direction of an ion which has been field evaporated. The ion is diverted from its initial direction originating from the specimen to a detector. The reflectron can increase the mass resolution of the three-dimensional atom probe in a similar way to its use in a TOF mass spectrometer. A conventional reflectron is formed of a series of ring electrodes, which define a hollow cylinder. The electrodes are each held at an electric potential, the potential increasing in a direction of travel of an ion from an ion source. The electrodes generate a uniform field over the cross-section of the reflectron.
The flatness of the fields can be a key design criterion for conventional reflectrons. Any residual curvature of the fields, which is difficult to avoid, leads to aberrations in ion trajectories and degradation in mass resolution. The ions travel in a parabolic path through the reflectron. Ions with more kinetic energy travel farther into the reflectron, hence their path length is longer and their transit time to the detector is longer. Ions with less kinetic energy do not travel as deep, traverse a shorter path, and have shorter transit times. It can be deduced that ions with a given mass-to-charge ratio and varying kinetic energies will have less variation in their transit time, hence the measured mass resolution will be improved. The reflectron can be configured such that the time taken by the ion to travel through the atom probe is substantially independent of the initial energy of the ion. This is known as time focusing. Conventional designs are limited in the acceptance angle over which they can work. Reflectrons are generally described in Cerezo et al., Rev. Sci. Instrum. 69 (1998) (see e.g., pages 49-58), which is fully incorporated herein by reference.
Ions liberated with the same mass-to-charge ratio but slightly different kinetic energies will follow different trajectories through the reflectron and will strike the detector at slightly different locations. The spread of impact positions is proportional to the chromatic aberration of the system. In addition, as the field of view (FOV) increases so does the chromatic aberration.
Laser pulses may be used instead of, or in addition to, voltage pulsing to stimulate ion evaporation. This is particularly useful for non-conducting or poorly conducting specimens such as semiconductors and oxides. Transient surface heating and bulk heating of the specimen, which effectively lengthens the period over which evaporation occurs, can degrade mass resolution. In some cases the high temperatures achieved lead to diffusion of atoms in the specimen and thus a loss of spatial resolution.
Laser pulsing is described in Liu, C. Wu and T. T. Tsong, Surface Science, 246 (1991) (see e.g., 157-162), which is fully incorporated herein by reference. High FW0.5M resolutions were obtained (only with long flight path and hence narrow FOV but Full width Tenth, Hundredth and Thousandth maximum resolutions were limited by exponential tails.